sprinter 02 2.1 313cdi?

can anyone advise or help i got 313 cdi 02 2.1 sprinter, it goes brilliant for 2-3 weeks then lets out little smoke engine goes noisy and goes into limp mode, i have replaced with genuine parts from merc. which includes fuel filter, pressure convertor valve, still no joy it wont take or go above 3000 revs,if i stop engine then restart it goes brilliant until 3- weeks then, i have tried loosening injectors one by one when engine running 3 stops the engine and has no or little diesel comming out other one has no affect it keeps running and has fuel running out could this be fault or please advise or help thanks